Nice boats, those Cav 39's, congratulations.
Even though the main is smaller than a big genoa
it is still a critical sail, for most wind
angles except downwind.
With the wind forward of the beam the main balances the boat. Without it speed might be similar but you will be more likely to have erratic steering
in gusty conditions. The main can easily be fine tuned to adjust balance, reducing the need for big rudder
For me, on any boat the main generally is the first sail up and the last down. Occasionally if my course is downwind I might just unroll the headsail, but normally I use both sails together.
Basically treat it the same as the noelex 25 for starter, but you will probably find she doesn't sail very well under main alone like the fractional boat might have. The big headsail can be a pain to tack. For close quarters manuvering under sail I use the full main and often roll half the genoa
away. This reduces speed, but enables me to back the headsail to push the bow around and improves visibility.
Generally I would reef the main first, then take a few rolls out of the genoa, then reef the main again. But sometimes reaching you can leave more headsail out and deep reef the main.
Downwind you can sheet the genoa off the end of the mainboom and drop the main. Sometimes I will run just with a preventered main and no headsails. No as nice steering
wise but its versitile if the wind shifts and saves having the genoa backwinds behind the main. On long downwind legs poling the genoa out to windward works very well.